Temperature protection method for motor vehicle engine axhaust pipe

ABSTRACT

The invention concerns a temperature protection method for a motor vehicle engine exhaust pipe, comprising steps which consist in determining an ignition advance angle among several possible values including a stall speed angle, based on a predetermined logic and operational parameters of the engine.

[0001] The invention relates to the overall field of ignition control inmotor vehicle engines. It deals more particularly with the protecting ofexhaust pipes, which are sensitive to overheating.

[0002] Most modern motor vehicles use, to reduce polluting emissions,exhaust systems of the catalytic type, which receive hot gases from theengine via the exhaust pipe.

[0003] These catalytic systems are particularly sensitive to excessiveoverheating, which destroys the chemical properties of the catalysts.

[0004] Now, it has been found that, for the phases of operation of theengine during which there is no desire to produce power, the ignitionangle (often denoted IGA) for nominal ignition passes to a minimumignition angle (which is close to stalling speed) and there is thereforea search to regulate the ignition event so that it takes place justearly enough to burn the air/fuel mixture in the pistons, but with acombustion which is slightly offset in time (in comparison to normaloperation) and produces no power.

[0005] This delayed combustion gives rise to a great dissipation of heatin the exhaust pipe. It is therefore essential to make sure that thetemperature reached TEG (temperature of exhaust gas) can under nocircumstances reach a limit which would cause the catalyst to bedestroyed.

[0006] Methods for controlling temperature in exhaust pipes which employreal-time measurement of the temperature at a point on the pipe andpossible injection of excess fuel into the mixture introduced in thepistons are known, this unburnt fuel (unburnt because it is in excess inthe mixture) being intended solely to cool the exhaust pipe.

[0007] As will be understood, such a system is not very economical withfuel, and devices have been proposed for reinjecting the mixture leavingthe catalytic system into the carburetor with a view to reducing thisloss of unburnt fuel. Clearly these devices are at once complex,difficult to regulate, and expensive.

[0008] In general, temperature protection of the exhaust pipe is alsoobtained by keeping a margin between the minimum ignition advance towhich the engine is set and the ignition advance that corresponds tostalling speed. In this way, the dissipation of heat into the pipe isreduced and this then affords protection against temperature. However,it is clear that this solution is barely satisfactory, in that itmaintains the production of a certain amount of power by the engine.

[0009] The present invention therefore proposes a novel method fortemperature-protecting an exhaust pipe.

[0010] According to the invention, the temperature protection method fora motor vehicle exhaust pipe comprises steps of determining a minimumignition angle from several possible values, including a stalling-speedangle, on the basis of predetermined logic and of engine operatingparameters.

[0011] As a preference, the minimum ignition angle may be set down aslow as the stalling speed for periods of time in which certain engineparameters lie within a determined range of values.

[0012] The description and the drawings which follow will make itpossible to gain a better understanding of the objects and advantages ofthe invention. Clearly, this description is given solely by way ofexample, and is not in any way limiting.

[0013] In the drawings:

[0014]FIG. 1 illustrates the power produced by a piston engine accordingto the ignition advance setting;

[0015]FIG. 2 shows a simplified flow diagram of the method according tothe invention;

[0016]FIG. 3 illustrates an example of the variation over time of theignition angle in the absence of hysteresis;

[0017]FIG. 4 illustrates, in the same way, an example of the variationover time of the ignition angle when hysteresis is present. Theinvention finds its niche in the context of a motor vehicle ofconventional type, with pistons and atmospheric exhaust pipe. Theseelements as such do not form part of the present invention, and areconsidered to be known to those skilled in the art.

[0018]FIG. 1 shows various curves of power as a function of engine speedN, each curve corresponding to a different setting of the ignitionangle. It is simply recalled here, using FIG. 1 for reference, that,when driving the vehicle, when the driver accelerates, the engine is setto supply a maximum production of power, which corresponds to a firstsetting of the ignition advance IGA_REF known as the pinging angle(point 1 on the curve labeled REF, which is the curve of maximum torqueproduced by the engine). This setting, which is optimal from the pointof view of the production of engine power, causes the engine to ping andthis leads to premature wear thereof, and, in practical terms, theignition angle is set to a second angle IGA_BASE (point 2 on the curvelabeled BASE) known as the nominal angle, which is therefore set backslightly from the pinging angle.

[0019] At a given moment in time, the angle actually chosen (point 5) isoften smaller than the nominal value, and naturally depends upon thetotal engine torque demand from the accelerator, the air-conditioning,the power-assisted steering, etc.

[0020] By contrast, when the driver lifts his foot off the acceleratorto slow down or to shift gear, the ignition angle is altered and in theprior art is generally brought down to a practical minimum valueIGA_MIN_TEG (point 4 on a curve known as MIN-TEG for temperature ofexhaust gas) for which power is still produced, without having excessiverisks of high temperature TEG in the pipe. In practice, this curve waskept very much above the stall-angle curve, so as to keep a wide safetymargin.

[0021] In the present invention, it is proposed for the ignition angleto be brought down, at least during certain periods, to a third valueknown as the stalled angle IGA_MIN (engine speed with no production ofpower, point 3 on the curve labeled MIN). For this value of angle, theproduction of engine torque is at a minimum and the engine is simplyturning over because it driven along by the motion of the vehicle. Thisangle simply makes it possible to ensure that the air-fuel mixture isburnt within the piston.

[0022] The present invention therefore uses the pinging angle, thenominal angle and the stalling angle, and the method consists inchoosing, at each moment (or at least iteratively) one of these anglesaccording to parameters concerned with driving and parameters concernedwith the operation of the vehicle (engine, pipe, etc).

[0023] It will be understood that, in principle, the method consists inbringing the ignition angle IGA down to a point on the MIN curve for aperiod of time which lasts for as long as the temperature TEG measuredor modeled at the inlet to the exhaust pipe or to the catalyticconverter does not exceed a given threshold. As soon as the measuredtemperature is above a certain threshold value, the ignition angle IGAis once again altered to reduce the production of heat. This brings itback to an angle value IGA placed on the MIN TEG curve that isintermediate between the stalling angle curve and the nominal curve.

[0024] As can be seen in FIG. 2, according to the method, a first stepE1 is used to calculate the values of the ignition angles correspondingto the REF, BASE and MIN curves using parameters concerned with theoperation of the engine at that moment. This step E1 is also used tocalculate an ignition angle MIN_TEG which takes account of thetemperature actually measured or modeled at the inlet to the catalyticconverter (or some other sensitive point elsewhere). This value dependson the conditions observed in the engine and on the environment at agiven moment, and is not necessarily definitively fixed at the outset.

[0025] Advantageously, as will be explained in greater detailhereinafter, this value is adapted to suit the temperature gradient TEGobserved when this ignition angle is used.

[0026] In a second step E2, conducted in parallel with step E1, thetemperature in the pipe is modeled or measured.

[0027] In a third step E3, also conducted at the same time as the stepsE1 and E2, the instantaneous torque demand is determined. Thisdemanded-torque value depends in particular on whether or not certaindevices in the vehicle, such as the air-conditioning or thepower-assisted steering, are in operation, on the position of theaccelerator pedal and on which gear is engaged, etc.

[0028] On the basis of this information and these parameters a step E4chooses the minimum ignition angle that can be used at that moment, andthat value is applied to the operating of the engine in step E5.

[0029] It is clear that, at the same time as altering this ignitionangle value, the injection and throttle setting values (each of whichalso has maximum REF, nominal BASE and minimum MIN values) are alsoaltered to achieve the desired torque (step E6)

[0030] However, the alteration to the ignition angle value can beperformed very quickly and has a very quick effect on the engine torque,in comparison to adjusting the injection or the throttle valve, whichare parameters that adjust slowly.

[0031] The module coordinating torque back-up and the torque-variationmodule (for example during a gearshift) both simultaneously use thecalculation of the minimum ignition angle, taking into consideration thepipe temperature in order to determine which engine parameters need tobe altered according to the current and predicted torque demand(particularly the predicted duration of the reduction in torque).

[0032] For example, during deceleration, the torque control computer TQM(the torque management system) can set the ignition angle to the minimumvalue. If this deceleration phase is extended, and if the minimum angle(IGA_MIN, on the MIN curve in FIG. 1) is smaller than the ignitionangle, taking into consideration the temperature of the gases(IGA_MIN_TEG, curve MIN_TEG in FIG. 1) the computer TQM will decide tolimit the reduction in ignition angle to a value close to IGA_MIN_TEG,and the rest of the demanded deceleration will be obtained by action onother engine parameters such as by cutting off the injection of fuel toone cylinder, etc.

[0033] The method is by nature iterative. As was stated, by way ofexample, if the driver lifts his foot off the accelerator pedal and inthe absence of other torque demands, the computer will bring theignition angle down to the value IGA_MIN for a few seconds and then,depending on the measured or modeled change in the value of thetemperature at the inlet to the pipe, will then choose an offset valueIGA_MIN_TEG for as long as necessary to bring down the temperature inthe pipe.

[0034] This situation is illustrated by FIG. 3. By way of example, themaximum temperature is not allowed to exceed 850° C. at the inlet to thepipe. In this case, the ignition angle is kept on the MIN curve until ameasured temperature point of 840° (or a value giving consideration to apredetermined threshold) is reached and then brought down onto a MIN_TEGcurve for as long as the temperature at the inlet to the pipe remainsclose to 850° C. Thereafter, successive jumps between the MIN TEG andMIN curves will be brought about, according to the temperature measured.

[0035] The minimum ignition angle that takes account of the pipetemperature (IGA_MIN_TEG) and the associated curve MIN_TEG arecalculated by a matching process as soon as the ignition angle used lieson the MIN_TEG curve. This matching process analyzes the temperaturegradient TEG and adjusts the value IGA_MIN_TEG to keep this gradient ata negative value. As a result, when the ignition angle is equal toIGA_MIN_TEG, the pipe temperature decreases to allow the ignition angleto revert to IGA_MIN. If the temperature gradient TEG is positive oreven zero, that is to say if the temperature TEG continues to rise orremains stable, then the curve MIN_TEG and therefore the ignition anglevalue IGA_MIN_TEG are altered in the direction of an increased ignitionangle (the curve MIN_TEG diverges from the curve MIN) until thetemperature TEG drops. The new value of the ignition angle IGA_MIN_TEGis then stored in memory to be used as the initial value during the nextjump between the MIN curve and the MIN_TEG curve.

[0036] When the pipe temperature no longer makes it possible for theignition angle to be kept at the stalling-angle value (MIN), theinformation on the angle actually chosen is used by the computer in stepE3 to alter other parameters of action on the engine (throttle,injection).

[0037] The method according to the invention is intended to beimplemented by an engine parameter computer, generally already presentin motor vehicles, of a nature and structure which are here consideredto be known per se.

[0038] It is clear that the choice of the pipe inlet temperature as aparameter to be monitored is merely one example. It would be just aspossible to decide to regulate the ignition advance using any otherrelevant parameter.

[0039] Likewise, the parameter for monitoring (temperature or some otherparameter) may naturally be either measured directly by an appropriatesensor or modeled by the engine computer, on the basis of other engineoperating information such as the inlet time, the inlet temperature, theinlet pressure, the amount of air in the pistons, the ignition angle,etc.

[0040] The same is true of the engine torque demand, which is eithermeasured, or also modeled according to the position of the acceleratorpedal and other parameters of the vehicle.

[0041] In an alternative form of embodiment, use is made of anintermediate variable TEG_HYS instead of the variable TEG for thetemperature of the exhaust gases, so as to take account, among otherthings, of the thermal inertia of the catalytic converter. This variablerepresents the temperature of the gases to which hysteresis is added.This makes it possible to anticipate overheating of the exhaust system(particularly of the catalytic converter) by reducing the time for whichthe ignition angle can remain at the stalling level IGA_MIN. When thetemperature TEG in the exhaust pipe increases, the variant TEG_HYSreaches the ignition angle setting-change threshold sooner. By contrast,when the temperature decreases, the variant TEG_HYS maintains for longera value that forces the computer to leave the ignition angle above theminimum ignition angle value IGA_MIN. This allows for better cooling ofthe exhaust pipe. In this way a curve of the change in temperature andof the change in ignition angle is obtained which is closer to the curveof FIG. 4 (compare to FIG. 3).

[0042] It will have been clearly understood that the use of thepreviously unused ignition angle margin is novel and can be achieved bymonitoring critical engine parameters and ongoing consideration of thetorque demand. The scope of the present invention is not limited to thedetails of the embodiments considered hereinabove by way of example, buton the contrary extends to cover modifications that are within thecompetence of the person skilled in the art.

1. A method for controlling a vehicle engine according to thetemperature of its exhaust pipe, of the type comprising the steps ofdetermining a minimum ignition angle from several possible valuescomprising a stalling-speed angle (MIN, MINTEG), on the basis ofpredetermined logic and of engine operating parameters, characterized inthat it includes a step in which the ignition angle (IGA) is returned toa stalling ignition angle (IGA_MIN) for a period which lasts as long asthe temperature (TEG) at the inlet to the exhaust pipe does not exceed agiven threshold, and, as soon as the temperature (TEG) exceeds a certainthreshold value, the ignition angle (IGA) is brought to a second value(IGA_MIN_TEG) that takes account of the exhaust temperature (TEG). 2.The method as claimed in claim 1, characterized in that it comprises thefollowing steps: E1 calculating a value for the minimum ignition angle(IGA_MIN) according to the engine operating parameters of the moment,and calculating an ignition angle (IGA_MIN_TEG) that takes account ofthe exhaust temperature, E2 determining the temperature (TEG) in thepipe, E3 determining the torque demand, E4 choosing the engineparameters to alter according to the torque demand and choosing theminimum ignition angle (IGA) value that can be used at that moment. 3.The method as claimed in the preceding claim, characterized in that thecalculation of the ignition angle (IGA_MIN_TEG) that takes account ofthe exhaust temperature includes a matching process able to alter saidvalue in such a way that the gradient of said exhaust temperature (TEG)is negative when said ignition angle (IGA_MIN_TEG) is used.
 4. Themethod as claimed in any one of the preceding claims, characterized inthat the temperature in the pipe is determined by direct measurement. 5.The method as claimed in any one of the preceding claims, characterizedin that the temperature in the pipe is determined by modeling.
 6. Themethod as claimed in claim 2, characterized in that, in step E4 ofchoosing the engine parameters to alter, it comprises a preferred choiceof altering the ignition angle and, when the exhaust temperature (TEG)exceeds a given value, altering injection and throttle setting values.7. The method as claimed in any one of claims 1 to 6, characterized inthat the method is iterative.
 8. The method as claimed in any one ofclaims 1 to 7, characterized in that use is made of an intermediatevariable (TEG_HYS) instead of the variable (TEG), which is thetemperature of the exhaust gases, this intermediate variable comprisinghysteresis with respect to the temperature (TEG) about a predeterminedvalue.